The present invention generally relates to IMPATT diodes and more particularly relates to improved techniques and circuits for controlling the temperature of IMPATT diodes during pulsed operations, and even more particularly relates to techniques for providing essentially constant pulsed power output from an IMPATT diode or diodes when operated over a wide range of temperatures.
In today's aviation industry, it is common for a single aircraft to be subjected to several extreme thermal conditions in a relatively short time interval. It is not uncommon for an aircraft to be flying at an altitude of 40,000 feet with an outside temperature of less than -55.degree. C., while only moments earlier it was waiting for a take-off clearance from a hot, humid airport runway. With the current aspirations for trans-atmospheric aircraft, these extreme vicissitudes in the ambient temperature will continue to confront avionics engineers with perplexing problems of increasing difficulty and importance.
One particular problem that is exacerbated by these temperature oscillations is the control of the junction temperature of IMPATT diodes used to generate microwave power in the GHz frequency range. Impact ionization avalanche transit time (IMPATT) diodes have many uses, but they are best known for their abilities to produce negative resistances and act as reliable solid-state sources of power at microwave frequencies. A similarity in all IMPATT diodes is the fact that they contain at least a single junction between a P-type semiconductor and an N-type semiconductor. A brief description of the operation, construction, and function of IMPATT diodes is given in Sze, Semiconductor Devices, Physics and Technology, published in 1985 by John Wiley and Sons, New York, N.Y., in chapter 6.2 at pages 229-234 (inclusive), which is hereby incorporated herein by this reference.
The power output, efficiency, and operating impedance of IMPATT diodes, which are used to generate microwave power, depend critically upon their instantaneous junction temperature. One example of the use of IMPATT diodes which creates a difficult junction temperature control situation is in airborne weather radar. In such applications, the IMPATT diodes are operated in a pulsed manner, thereby causing a fluctuation of the junction temperature. For example, diodes which are tuned at room temperature for a 30 usec pulse width, typically tend to lose approximately 1 to 2 db of average pulse power when operated at +85.degree. C. and greater than roughly 3.0 db of pulse power at -55.degree. C. Due to the junction heating, the slope of power output versus time plot across the pulse often becomes much steeper at low temperatures and can result in a power output change of greater than approximately 6.0 db for short pulses of 6.0 usecs or less. Efficiencies of roughly 20% at room temperature often can drop to less than 5% at -55.degree. C.
Several alternative techniques have been developed and implemented in the past to attempt to overcome these temperature variations. One method which has been used is, to place the chassis containing the IMPATT diodes in a controlled temperature environment. Some type of electrical heater or cooler, or both, is then implemented to maintain the IMPATT diodes in an optimum temperature range. Another method for maintaining IMPATT performance over temperature deals with an electronic preheat. This technique heats the diodes with a low magnitude, pulsed, reversed-biased avalanche current prior to the main pulse. The pulse is typically between 10 and 100 usec's in width, and usually less than 100 milliamps in magnitude for each diode. This technique has been employed in some existing avionics weather radars.
While these techniques, or variations of them, have been used for controlling the temperature of IMPATT diodes during pulsed operations, they do have numerous and serious drawbacks. One major problem with the environment temperature control method is that it often results in low heating efficiency and even lower cooling efficiency of the large chassis: The typical thermal time constant of several minutes associated with bringing the IMPATT devices up to operating temperature, the moisture condensation and evaporation due to maintaining the IMPATT chassis at some temperature different from the ambient, and the size and bulk of the heaters, coolers and associated cables necessary to implement such a temperature control method are further problems.
A major downfall of the second technique is that the magnitude of the preheat current is not great enough to produce significant junction heating at very cold temperatures. Typically a current of five to ten times larger would be required to produce the desired heating. Any attempts, however, to increase the preheat current may produce free-running or locked oscillations from the diode which might be transmitted into space via the antenna. This is undesirable for short ranges (with short pulses) due to the loss of distance resolution and is undesirable for long ranges (with long pulses) due to the loss of storm feature resolution. Increasing the width of the preheat pulse is ineffective due to the typical 100 usec thermal time constant of the typical IMPATT chip.
Consequently, a need exist for improvement in techniques and circuits for controlling the junction temperature of IMPATT diodes using a pulse operation to generate microwave power which are utilized in a wide range of ambient temperatures.